Recently, in magnetic recording-reproducing systems for computer data, thin-film magnetic heads have been practically used. Since the thin-film magnetic heads can be easily downsized and applied to multi-track heads, they are widely used for multi-track linear recording systems, particularly, employing magnetic recording tapes as recording media. The downsized head elevates the track density to improve the recording efficiency, and consequently enables the recording tape to record the data in a high density. Further, in a multi-tracking system, it can increase the data transfer rate.
The thin-film magnetic heads are roughly categorized into two types, namely, magnetic inductive head (which responds to time-dependent variation of magnetic flux) and magnetoresistive head (which responds to scale of magnetic flux). Since the inductive head generally has a flat structure, its head coil consists of a relatively small number of turns. Accordingly, the inductive head cannot give magnetism large enough to obtain satisfying reproduction output. For this reason, the magneto-resistive head (MR head) giving enough output power, is used for reproduction, while the inductive head is used as a recording head. Those magnetic heads are generally incorporated in a unified system adopting the linear recording method for rapid data transfer.
The magnetic recording tape used in the system equipped with an MR head is determined depending on the recording and reproducing system. For example, magnetic recording tapes applied to IBM systems of 3480, 3490, 3590 and 3570 are known and commercially available. Each of those tapes has a basic structure in which a single thick magnetic layer (thickness: approx. 2.0 to 3.0 .mu.m) containing a ferromagnetic powder and a binder is provided on a flexible support. In general, in order to ensure good running durability and to inhibit wrong winding, the tape for recording computer date further comprises a back-coating layer provided on the reverse side (i.e., side on which the magnetic recording layer is not provided) of the support.
It is a problem that the magnetic recording tape having the single thick magnetic layer can not always store a great deal of date. Therefore, a magnetic recording tape having double layers has been proposed [Japanese patent provisional Publication No. 8(1996)-227517, which corresponds to Y. Kakuishi et al., U.S. Pat. application Ser. No. 08/602,567, filed Feb. 14, 1996]. In the proposed magnetic recording tape, a lower non-magnetic layer comprising a non-magnetic inorganic powder dispersed in a binder and an upper magnetic recording layer comprising a ferromagnetic powder dispersed in a binder are superposed and provided on a non-magnetic support. The upper magnetic layer can be made thin, and thereby output depression caused by thickness loss can be avoided. Further, the magnetic recording tape having double layers can store the computer data much more than a magnetic recording tape having a single thick layer because the data recording density can be increased. As an example, the aforementioned publication discloses a magnetic recording tape comprising a support made of polyethylene terephthalate (thickness: 10 .mu.m), and on one side of the support, a non-magnetic layer (thickness: 2.7 .mu.m) and a magnetic recording layer (thickness: 0.3 .mu.m) superposed in order.
For accomplishing both of high recording density and large recording capacity, a narrow magnetic recording tape is often used in the system adopting the linear recording method. In the system, it is necessary to precisely control the position of the magnetic head because the head is moved laterally (i.e., wide direction which is perpendicular to the longitudinal direction of the recording tape) to select the track for recording or reproducing.
In a conventional system of linear recording method, the magnetic recording tape runs through a predetermined path guided by the support members (e.g., guide poles) and the magnetic head is laterally moved between the predetermined positions on the tape. If the narrow recording tape expands or shrinks with heat or moisture or if the recording tape runs with drift, the reproducing head can hardly trace the proper position of the track and consequently gives poor output. For avoiding this trouble, a magnetic recording tape on which servo signals are longitudinally recorded has been recently proposed. The servo signals precisely give the width position of the head on the recording tape, so that the head can be controlled to place at the proper position of the track. The servo signals consist of plural servo bands each of which comprises signals varying in the width direction, and hence the position of the head can be precisely detected by reproducing the signals. In that system, the width of the servo band and the lateral interval between the servo bands are preferred not to vary with heat, moisture, tape running and so on. Therefore, the magnetic recording tape bearing the servo signals is preferred to keep the constant lateral dimension even if the environmental conditions (e.g., temperature, humidity) vary.
Further, the magnetic recording tape used in the above-mentioned system is also desired to keep the constant longitudinal dimension while it repeatedly runs with a high speed. In the system equipped with an MR head (which can densely record the data), the recording tape is tightly stretched so as to ensure good contact between the recording tape and the head. Particularly at a moment to stop or start the recording tape, high tension is given to both ends of the tape [BOT: beginning (starting end) of the tape and the EOT: end (terminal end) of tape]. Consequently, the tension often longitudinally enlarges the tape to lower the output, and at the same time, to change the lateral dimension in the width direction. In that case, the servo signals recorded on the tape cannot precisely indicate the position of the head, and hence troubles are liable to occur. For this reason, the magnetic recording tape used in the above-identified system is desired to have improved mechanical strength also in the longitudinal direction.
According to the study of the inventors, the magnetic recording tape described in the aforementioned Japanese patent provisional Publication No. 8(1996)-227517 ought to be improved in the tracking and running characteristics for using for the system with an MR head. The magnetic recording tape considerably expands or shrinks with heat or moisture in the lateral direction, and hence the tracking accuracy often reduces to give poor output even if the tracking control based on the servo signals is conducted. Further, the magnetic recording tape having repeatedly run often causes troubles with the system.
It is an object of the present invention to provide a magnetic recording tape favorably employable for a magnetic recording-reproducing system employing a magnetoresistive head in the linear recording method. In more detail, the magnetic recording tape to be provided by the invention keeps the constant lateral dimension so that the magnetic head can trace the proper position of the track even if the recording tape has a narrow width.